In B-mode medical ultrasound imaging, the scanner transmits ultrasonic bursts into the body and detects the energy of ultrasonic echoes backscattered from both stationary and moving tissues in the body. Ultrasonic scanners may also offer two dimensional Doppler flow detection (also referred to as color Doppler imaging) to detect and image moving blood. Recently, some investigators have proposed using color Doppler blood flow imaging capabilities to image moving structures other than blood, for example the moving heart structure.
In B-mode imaging, stationary and moving targets are both imaged. Often these images are degraded by the presence of stationary noise or clutter from both electrical sources and acoustic sources. In cardiac ultrasound applications, the moving myocardium is of principal interest, and can be obscured by stationary noise detected with B-mode imaging. Color Doppler imaging methods will remove this stationary clutter but cannot detect moving myocardium with the required sensitivity and specificity, since Doppler processing has been optimized for detecting blood velocities and not slower moving myocardium. The characteristics of ultrasonic signals returning from moving myocardium are different from those signals returning from blood. For example, in blood flow detection, the processor recognizes echoes from the strong, slow moving heart tissue as clutter and removes them from the image by means of a stationary target canceller or a high pass filter or both. Such a high pass filter is illustrated in the system described in U.S. Pat. No. 5,014,710 entitled "Steered Linear Color Doppler Imaging" for the color Doppler path. It is precisely those strong, slow moving targets that correspond to moving myocardium, but they, too, are removed from the color Doppler image. Therefore, prior art color Doppler imaging systems have had difficulty imaging slow moving myocardium.